Angiogenesis, the process of forming new blood vessels, is critical in many normal and abnormal physiological states. Under normal physiological conditions, humans and animals undergo angiogenesis in specific and restricted situations. For example, angiogenesis is normally observed in wound healing, fetal and embryonic development and formation of the corpus luteum, endometrium and placenta.
Undesirable or inappropriately regulated angiogenesis occurs in many disorders, in which abnormal endothelial growth may cause or participate in the pathological process. For example, angiogenesis participates in the growth of many tumors. Deregulated angiogenesis has been implicated in pathological processes such as rheumatoid arthritis, retinopathies, hemangiomas, and psoriasis. The diverse pathological disease states in which unregulated angiogenesis is present have been categorized as angiogenesis-associated diseases.
Both controlled and uncontrolled angiogenesis are thought to proceed in a similar manner. Capillary blood vessels are composed primarily of endothelial cells and pericytes, surrounded by a basement membrane. Angiogenesis begins with the erosion of the basement membrane by enzymes released by endothelial cells and leukocytes. The endothelial cells, which line the lumen of blood vessels, then protrude through the basement membrane. Angiogenic factors induce the endothelial cells to migrate through the eroded basement membrane. The migrating cells form a “sprout” protruding from the parent blood vessel, where the endothelial cells undergo mitosis and proliferate. Endothelial sprouts merge with each other to form capillary loops, creating the new blood vessel.
Agents that inhibit angiogenesis have proven to be effective in treating a variety of disorders. Avastin™ (bevacizumab), a monoclonal antibody that binds to Vascular Endothelial Growth Factor (VEGF), has proven to be effective in the treatment of a variety of cancers. Macugen™, an aptamer that binds to VEGF has proven to be effective in the treatment of neovascular (wet) age-related macular degeneration. Antagonists of the SDF/CXCR4 signaling pathway inhibit tumor neovascularization and are effective against cancer in mouse models (Guleng et al. Cancer Res. 2005 Jul. 1; 65(13):5864-71). The isocoumarin 2-(8-hydroxy-6-methoxy-1-oxo-1H-2-benzopyran-3-yl) propionic acid (NM-3) has completed phase I clinical evaluation as an orally bioavailable angiogenesis inhibitor. NM-3 directly kills both endothelial and tumor cells in vitro and is effective in the treatment of diverse human tumor xenografts in mice (Agata et al. Cancer Chemother Pharmacol. 2005 December; 56(6):610-4.). Thalidomide and related compounds have shown beneficial effects in the treatment of cancer, and although the molecular mechanism of action is not clear, the inhibition of angiogenesis appears to be an important component of the anti-tumor effect (see, e.g., Dredge et al. Microvasc Res. 2005 January; 69(1-2):56-63). The success of TNF-alpha antagonists in the treatment of rheumatoid arthritis is partially attributed to anti-angiogenic effects on the inflamed joint tissue (Feldmann et al. Annu Rev Immunol. 2001; 19:163-96). Anti-angiogenic therapies are widely expected to have beneficial effects on other inflammatory diseases, particularly psoriasis. Although many anti-angiogenic agents have an effect on angiogenesis regardless of the tissue that is affected, other angiogenic agents may tend to have a tissue-selective effect.
Pericytes are one of the component cell types of the vasculature, along with endothelial cells and smooth muscle cells. The ratio of pericytes to endothelial cells is greatest in the central nervous system, and pericytes are thought to play a significant role in the blood-brain barrier. Diabetic retinopathy is marked by a loss of pericytes in the retinal microvasculature, and this loss of pericytes is thought to have an important pathophysiological effect on the progression of the disease.
It is desirable to have additional compositions and methods for inhibiting angiogenesis and for increasing pericyte coverage in vascularized tissues. These include methods and compositions which can inhibit the unwanted growth of blood vessels, either generally or in certain tissues and/or disease states.